Orthogonal training signals for transmission in an antenna array

ABSTRACT

A method and apparatus for generation of orthogonal training signals for transmission in an antenna array are described. In this embodiment, a set of P training signals is generated. The generation of the P training signals includes generating a first set of Zadoff-Chu sequences, where the first set of sequences is based on a first reference Zadoff-Chu sequence and (P−1) first subsequent Zadoff-Chu sequences, where each one of the first subsequent Zadoff-Chu sequences is a cyclic shift of the first reference Zadoff-Chu sequence. A second set of sequences is generated based on a second reference Zadoff-Chu sequence and (P−1) second subsequent sequences that are cyclic shift of the second reference sequence. The P training signals are determined based on the first set of sequences and the second set of sequences. The training signals are then transmitted through a plurality of transmit paths of a base station towards a wireless network.

TECHNICAL FIELD

Embodiments of the invention relate to antenna array systems; and morespecifically, to generation of orthogonal training signals fortransmission in an antenna array.

BACKGROUND

Antenna arrays have been widely used in wireless mobile networks fordirectional signal transmission and reception with an increased gaincompared to an omni-directional antenna. The increased gain translatesinto a higher cell density and data throughput. The sub-arrays of anantenna array are coupled with respective transmit paths within a basestation for receiving signals to be transmitted to a wireless network.The transmit paths are calibrated to remove any linear phase and/oramplitude distortions (hereafter simply referred to as phase distortion)that occurs in these paths. If the transmission beam pattern is out ofphase or otherwise phase-distorted, the signal transmitted by theantenna array of the base station (e.g., a radio base station (RBS)) atnormal transmission power may not be correctly received and decoded by auser terminal. To compensate for the phase distortions, the base stationmay transmit data at a higher power level; however, increasing thetransmission power acts as a load to the system, causing a reduction tothe power that can be allocated to other terminals. In addition, thesignal transmitted at higher power may interfere with other terminals,causing a reduction in signal quality. In addition, a status of thetransmit paths and their associated sub-arrays can be determined duringa “branch supervision” process. Branch supervision enables thedetermination of one or more metrics for detection of faults in atransmit path.

Several techniques exist for antenna array calibration and branchsupervision. In some techniques, special training signals are speciallyselected to be injected into the antenna array to perform thecalibration and/or the branch supervision. In these techniques, thesignals are selected with controlled and known signal properties. Someof these techniques supervision rely on the analysis of a feedbacksignal that is a combination of signals after they have traversed thetransmit paths of the base station. In these techniques, thedetermination of the impairment function that affects the signals withinthe transmit paths (prior to calibration) or the determination of themetrics used for fault detection in the transmit paths use the samplingand generation of a set of linear equations.

To solve the set of linear equations an inversion of a matrix or pseudomatrix inversion is performed. In Advanced Antenna Systems (AAS), wherethe number of sub-arrays is large (e.g., an antenna may include 16sub-arrays, 32 sub-arrays, or more) the matrix inversion or pseudoinversion can be computational and memory intensive. In addition, insome techniques, the training signals used for performing thecalibration or branch supervision need to be stored in memory at thebase station. Thus, depending on the number of transmit paths of thebase station the number training signals can be large requiring asignificant amount of storage size at the base station.

SUMMARY

Embodiments of the invention enable the generation of orthogonaltraining signals for transmission in an antenna array of a base station.The antenna array includes a plurality of sub-arrays coupled to transmitpaths for transmitting outbound traffic signals to a wireless network.The orthogonal training signals can be used to perform calibration ofthe transmit paths of the base station and/or branch supervision of eachtransmit path.

One general aspect includes a method in a base station including aplurality of P transmit paths coupled with a plurality of sub-arrays ofan antenna array for transmitting signals to a wireless network, themethod including: generating a set of P training signals, wheregenerating the set of P training signals includes: generating a firstreference Zadoff-Chu sequence, where the first reference Zadoff-Chusequence is of length N, where N indicates a number of fast Fouriertransform (FFT) bins and N is a prime number that is greater than orequal to the number P of transmit paths; generating (P−1) firstsubsequent Zadoff-Chu sequences, where each one of the first subsequentZadoff-Chu sequences is a cyclic shift of the first reference Zadoff-Chusequence, and where a cyclic shift between two consecutive sequencesfrom the first set is an integer that is smaller than or equal to thenumber N of FFT bins divided by the number P of transmit paths, wherethe first reference Zadoff-Chu sequence and the (P−1) first subsequentZadoff-Chu sequences form a first set of P first sequences; generating asecond reference Zadoff-Chu sequence of length Neq, where Neq indicatesa number of FFT frames and Neq is a prime number that is greater than orequal to the number P of transmit paths; generating (P−1) secondsubsequent Zadoff-Chu sequences, where each one of the second subsequentZadoff-Chu sequences is a cyclic shift of the second referenceZadoff-Chu sequence, and where a cyclic shift between two consecutivesequences from the second set is an integer that is smaller than orequal to the number of FFT frames Neq divided by the number P oftransmit paths, where the second reference Zadoff-Chu sequence and the(P−1) second subsequent Zadoff-Chu sequences form a second set of Psecond sequences; and determining the set of P training signals based onthe first set of first sequences and the second set of second sequences.The method also includes transmitting each one of the P training signalsthrough a transmit path from the plurality of transmit paths of the basestation towards a wireless network.

One general aspect includes a base station including a plurality of Ptransmit paths coupled with a plurality of sub-arrays of an antennaarray for transmitting signals to a wireless network, the base stationincluding: an orthogonal training signal generator that is operative to:generate a set of P training signals, where to generate the set of Ptraining signals includes: to generate first reference Zadoff-Chusequence, where the first reference Zadoff-Chu sequence is of length N,where N indicates a number of fast Fourier transform (FFT) bins and N isa prime number that is greater than or equal to the number P of transmitpaths; to generate (P−1) first subsequent Zadoff-Chu sequences, whereeach one of the first subsequent Zadoff-Chu sequences is a cyclic shiftof the first reference Zadoff-Chu sequence, and where a cyclic shiftbetween two consecutive sequences from the first set is an integer thatis smaller than or equal to the number N of FFT bins divided by thenumber P of transmit paths, where the first reference Zadoff-Chusequence and the (P−1) first subsequent Zadoff-Chu sequences form afirst set of P first sequences; to generate a second referenceZadoff-Chu sequence of length Neq, where Neq indicates a number of FFTbins frames and Neq is a prime number that is greater than or equal tothe number P of transmit paths; to generate (p−1) second subsequentZadoff-Chu sequences, where each one of the second subsequent Zadoff-Chusequences is a cyclic shift of the second reference Zadoff-Chu sequence,and where a cyclic shift between two consecutive sequences from thesecond set is an integer that is smaller than or equal to the number ofFFT frames Neq divided by the number P of transmit paths, where thesecond reference Zadoff-Chu sequence and the (P−1) second subsequentZadoff-Chu sequences form a second set of P second sequences; and todetermine the set of P training signals based on the first set of firstsequences and the second set of second sequences. The orthogonaltraining signal generator is also operative to transmit each one of theP training signals through a transmit path from the plurality oftransmit paths of the base station towards a wireless network.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that differentreferences to “an” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone. Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is submitted that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

FIG. 1 illustrates a block diagram of an exemplary network architecturein accordance with some embodiments.

FIG. 2 illustrates a block diagram of an exemplary orthogonal trainingsignals generator for generating orthogonal training signals inaccordance with a first embodiment.

FIG. 3 illustrates exemplary P training signals obtained as a result ofthe replication operation a first set of ZC sequences and themultiplication of the replicated ZC sequences with a second set of ZCsequences in accordance with some embodiments.

FIG. 4A illustrates a flow diagram of exemplary operations performed forgenerating orthogonal training signal in accordance with a firstembodiment.

FIG. 4B illustrates a flow diagram of exemplary operations that areperformed for determining the P training signals based on the first ZCsequences and the second ZC sequences in accordance with someembodiments.

FIG. 4C is a flow diagram of exemplary operations that can be performedduring generation of the P training signals in accordance with someembodiments.

FIG. 5 illustrates a block diagram of an exemplary orthogonal trainingsignals generator that is operative to generate P training signals thatare orthogonal to one another in the frequency domain in accordance withsome embodiments.

FIG. 6 illustrates a block diagram of an exemplary orthogonal trainingsignals generator that is operative to generate P training signals thatare orthogonal to one another in the time domain in accordance with someembodiments.

FIG. 7 illustrates an exemplary block diagram of a portion of atransmitter in a base station that performs antenna array calibrationand/or branch supervision according to some embodiments.

FIG. 8 illustrates a flow diagram of exemplary operations for using thetraining signals to perform calibration and/or branch supervision of thetransmit paths of a base station in accordance with some embodiments.

DESCRIPTION OF EMBODIMENTS

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description. It will beappreciated, however, by one skilled in the art, that the invention maybe practiced without such specific details. Those of ordinary skill inthe art, with the included descriptions, will be able to implementappropriate functionality without undue experimentation.

In the following description and claims, the terms “coupled” and“connected,” along with their derivatives, may be used. It should beunderstood that these terms are not intended as synonyms for each other.“Coupled” is used to indicate that two or more elements, which may ormay not be in direct physical or electrical contact with each other,co-operate or interact with each other. “Connected” is used to indicatethe establishment of communication between two or more elements that arecoupled with each other.

Embodiments of the invention provide for orthogonal training signalsthat are generated for transmission from a base station through anantenna array including multiple sub-arrays. In the followingdescription, the orthogonality of the training signals can be obtainedin the time domain, in the frequency domain, and/or both the time andthe frequency domains. In some embodiments, the training signals can beused for performing an efficient calibration of the antenna array in thetransmit direction. In other embodiments, the training signals can beused to perform branch supervision of each of the transmit path coupledwith sub-arrays of the antenna array. In some embodiments, the same setof signals can be used for performing calibration and branchsupervision. In other embodiments, different sets of signals may begenerated for performing calibration and branch supervisionindependently. As will be apparent from the following description, thisapproach allows for a more efficient calibration and branch supervisionby minimizing the computational and memory resources needed forgeneration and processing of training signals that are during thecalibration and/or the branch supervision processes.

According to a first embodiment, the training signals generated areorthogonal in the frequency domain as well as in the time domain. Thesesignals can be referred to as time-frequency orthogonal trainingsignals. In this embodiment, a set of P training signals is generated.The generation of the P training signals includes generating a firstreference Zadoff-Chu sequence, where the first reference Zadoff-Chusequence is of length N, where N indicates a number of Fast FourierTransform (FFT) bins and N is a prime number that is greater than orequal to the number P of transmit paths; generating (P−1) firstsubsequent Zadoff-Chu sequences, where each one of the first subsequentZadoff-Chu sequences is a cyclic shift of the first reference Zadoff-Chusequence, and a cyclic shift between two consecutive sequences from thefirst set is an integer that is smaller than or equal to the number N ofFFT bins divided by the number P of transmit paths, where the firstreference Zadoff-Chu sequence and the (P−1) first subsequent Zadoff-Chusequences form a first set of P first sequences. The generation of thetraining signals further includes: generating a second referenceZadoff-Chu sequence of length Neq, where Neq indicates a number of FFTframes and Neq is a prime number that is greater than or equal to thenumber P of transmit paths, generating (P−1) second subsequentZadoff-Chu sequences, where each one of the second subsequent Zadoff-Chusequences is a cyclic shift of the second reference Zadoff-Chu sequence,and where a cyclic shift between two consecutive sequences from thesecond set is an integer that is smaller than or equal to the number ofFFT frames Neq divided by the number P of transmit paths, where thesecond reference Zadoff-Chu sequence and the (P−1) second subsequentZadoff-Chu sequences form a second set of P second sequences. Thegeneration of the training signals further includes determining the setof P training signals based on the first set of sequences and the secondset of sequences. The training signals are then transmitted through aplurality of transmit paths of a base station towards a wirelessnetwork. The training signals can be used to perform branch supervisionand/or calibration of the transmit paths within the base station.

According to some embodiments, the training signals are generated to beorthogonal in the frequency domain only. In other embodiments, thetraining signals are generated to be orthogonal in the time domain only.

The embodiments described herein enable calibration techniques andbranch supervision techniques that have several advantages when comparedwith existing calibration techniques. The use of the orthogonal trainingsignals enables the elimination of the inversion of matrix or pseudoinversion of matrix that was previously performed for the solution ofthe transfer function saving computation time and memory. Further, onlytwo sets of Zadoff-Chu sequences are used to solve the transfer functionof all the transmit paths which greatly simplifies the implementation ofthe impairment function estimation and minimize the memory needed. Theorthogonality of the training signals provides improved measures ofsignal-to-interference-plus-noise ratio (SINR) for the estimation of thetransfer function. The set of training signals can be used for bothantenna calibration and branch supervision. Each of the training signalshas a low peak to average power ratio (i.e. the signals have a wellbehaved time response when used in a transmit path). In someembodiments, the training signals are generated in the frequency domain,which ensures a good signal level for a transfer function estimationacross frequency.

FIG. 1 illustrates a block diagram of an exemplary network architecturein which an embodiment of the invention may operate. A base station 110,such as an RBS, is coupled to one or more network nodes 120A-N (e.g.,other base stations) and/or one or more user equipments 140A-M (e.g.,mobile phones, tablets, Internet of Things (IoT) devices, etc.) via awireless network 130. The wireless network 130 operates in compliancewith a wireless communication standard, such as 5G, 4G, LTE, GSM, CDMA,WCDMA, etc. The base station 110 includes a receiver 112, a transmitter114, both of which are coupled to an antenna 118 for signal transmissionand reception. The receiver 112 and the transmitter 114 may also becoupled to a controller 116 that controls the transmission and receptionoperations. It is understood that the base station 110 of FIG. 1 is asimplified representation; additional circuitry may be included in abase station that performs the antenna array calibration describedherein. The transmitter 114 includes an orthogonal training signalsgenerator (OTSG) 150 that is operative to generate a set of P trainingsignals that are orthogonal to one another. The orthogonality of thetraining signals can be achieved in the frequency domain only, in thetime domain only or in both the frequency and the time domainssimultaneously. The generated training signals can be used to performcalibration and/or branch supervision within the transmit paths of thebase station.

The transmitter 114 includes ports multiple transmit paths (notillustrates in FIG. 1). The term “transmit path” as used herein refersto the path traversed by a signal after the signal enters a transmit(Tx) chain and before the signal enters one or more sub-arrays from theantenna 118. In some embodiments, each transmit path is coupled with asingle sub-array of the antenna 118. In other embodiments, a transmitpath may be spitted to multiple sub-arrays of the antenna 118. Inpractice a transmit path may also include duplexers, amplifiers (e.g.,tower mounted amplifiers (TMAs), combiners, diplexers, etc., such aswould be appreciated by one skilled in the art. There is a one-to-onecorrespondence between a transmit chain and a transmit path. Thetransmit chains are the boundary between digital processing and analogprocessing in the base station 110, as each one of the transmit chainsconverts a signal from digital to analog. Each one of the transmitchains includes a number of analog components, such as one or moredigital-to-analog converters, mixers, filters, power amplifiers, etc.The analog components in the transmit chains, together with the feedersand other components along the analog portion of the transmit paths upto the antenna ports, generally cause linear phase and/or linearamplitude impairment to the signals that traverse these paths. Anexample of a transmit path is shown in FIG. 7 by the dotted box labeledas a transmit path 729-1. In this example, the transmit path 729-1includes a transmit (Tx) chain 730-1 and all of the interconnectincluding a feeder (not illustrated) up to a coupler 723-1 inside theantenna array 118.

FIG. 2 illustrates a block diagram of an exemplary orthogonal trainingsignals generator (OTSG) 250 used for generating orthogonal trainingsignals in accordance with a first embodiment. In this first embodiment,the generated orthogonal training signals are P signal that areorthogonal to one another in both the time and the frequency domains.

The OSTC 250 includes a first reference Zadoff-Chu (ZC) sequencegeneration unit 201-1. The first reference ZC generation unit 201-1 isoperative to generate a first reference ZC sequence Z₁ of length N, withparameters u₁ and q₁. In some embodiments, the sequence Z₁ is generatedaccording to the following equation (1):

$\begin{matrix}{Z_{1} = {e^{({{- j}\frac{\pi({{u_{1}{\lbrack{0:{N - 1}}\rbrack}}{({{\lbrack{0:{N - 1}}\rbrack} + 1 + {2q_{1}}})}}}{N}})}.}} & {{equation}\mspace{14mu} (1)}\end{matrix}$

where N indicates a number of Fast Fourier Transform (FFT) bins selectedfor the sequence Z₁. In some embodiments, the number N is a prime numberthat is greater or equal to the number P of transmit paths in thetransmitter 118. In some embodiments, the first parameters u₁ is aninteger than is smaller than N, 0<u₁<N, and q₁ is any integer number.The sequence Z₁ is a signal generated at the first reference ZCgeneration unit 201-1 according to equation (1).

The sequence Z₁ is used as a reference signal to generate additional(P−1) ZC sequences by cyclically shifting Z₁ with a shift step size:

${{Step_{size}} = \left\lfloor \frac{N}{P} \right\rfloor},$

where └N/P┘ is the floor function applied to N/P, N is a prime numberthat is greater or equal to the number P of transmit paths of thetransmitter 118, and P is the number of transmit paths. The cyclic shiftis applied by each one of the Shift units 201-2 to 201-P to generate the(P−1) ZC sequences based on the first reference Z₁ sequence. The cyclicshift between two consecutive sequences from the first set of sequences(where the first set of sequences includes the first reference sequenceand the additional ZC sequences) is an integer that is smaller than orequal to the number N of FFT bins divided by the number P of transmitpaths.

Thus, for each i an integer between 2 and P, the sequence Z_(i) isgenerated based on the following equation (2):

Z _(i)=circular_shift(Z ₁ ,i*└N/P┘)  equation (2)

Where └N/P┘ is the floor function as applied to N/P and the result ofthe floor function is an integer that is smaller than or equal to thenumber N of FFT bins divided by the number P of transmit paths.

In some embodiments, when the sequences Z₁ . . . Z_(P) from the firstset of first sequences are generated in the frequency domain (as perequation (1) and equation (2)) these sequences are transformed into thetime domain by applying an Inverse FFT (IFFT) over the N FFT bins. TheIFFT is applied by the IFFT unit 205-i to each corresponding Z_(i)sequence in the frequency domain to obtain the sequence z_(i) in thetime domain Thus, the following set of sequences is generated:

z _(i)=IFFT(Z _(i)),  equation (3)

where IFFT is over N FFT bins.

In other embodiment, the first set of sequences is generated in the timedomain (e g, referred to as sequences z_(i)) and the operation ofapplying an IFFT on the signals is not performed. In this embodiment,the units 205-1 to 205-P may not be implemented in the OTSG 250 oralternatively they may be disabled such that the sequences z_(i) are notprocessed at these units. The first reference sequence z₁ is a ZCsequence of parameters u₁ and q₁ is generated in the time domain. Thecircular shift applied by each one of the Shift units 201-1 to 201-P isa shift in the time domain and results in obtaining a set of ZCsequences z_(i), for i=2 . . . P, in the time domain. In the embodimentsdescribed herein upper case variables (such as Z_(i)) are used to referto signals that are expressed in the frequency domain, while lower casevariables (such as z_(i)) are used to refer to signals that areexpressed in the time domain.

In some embodiments, prior to determining the P training signals basedon the first set of sequences {z_(i), i=1 . . . P} and a second set ofsequences {D_(i), i=1 . . . P} several operations can be performed oneach one of the first sequences z_(i). These additional operations areperformed for each sequence z_(i) by a respective one of the power boostunits 202-i, the bandwidth limiters 203-i, the DC removal units 204-i,the cyclic prefix adder 206-i, and the power normalizer 207-i. In someembodiments, none of these operations is performed. In otherembodiments, a subset or all the operations are performed withoutdeparting from the scope of the present invention.

In the following description, the operations will be described withrespect to a single sequence Z_(i), however one of ordinary skill in theart would understand that the same operations are performed for each oneof the first sequences Z_(i), for i=1 . . . P. In the power boost unit202-i, the power level of the some of the FFT bins of the sequence Z_(i)are boosted to improve the signal to noise ratio of those bins. Forexample, in some embodiments, the FFT bins of the sequence that areclose to the edges of the bandwidth occupied (Noc) are boosted.

The sequence Zi is then transmitted to the bandwidth limiter 203-i thatis operative to limit the frequency band of the sequence based on theNoc. As mentioned above, Noc indicates the number of FFT bins of thesequence that are within the bandwidth of a digital transmit path. Thenumber Noc is usually narrower than the Nyquist bandwidth. In the firstembodiment, when the sequence z_(i) is generated in the time domain, thefrequency band can be limited by applying a digital filter with adesired bandwidth in the time domain. Alternatively, when the sequenceZ_(i) is generated in the frequency domain, the frequency band can belimited by applying a digital filter with a desired bandwidth in digitalfrequency domain. For example, this can be performed by nullifying FFTbins of the sequence Z_(i) that are outside of the occupied bandwidthand keeping the remaining of the FFT bins unchanged.

In some embodiments, the bandwidth limited 203-i limits the frequencyband of the sequence Z_(i) for i=1 . . . P to contain Noc occupied FFTbins according to the following equations:

The Noc is determined based on the following equation (4)

Noc=┌Carrier_Bandwith*s/Nyquist Sampling_Rate*N)┐   equation (4),

where the carrier_bandwitdh is the bandwidth of the carrier wave of thetransmit path, and s is a parameter that can have a value of 0.9. Othervalues of s can be contemplated.

Depending on whether Noc is even or odd, the sequence frequency band ofthe sequence Z_(i) is limited according to equation (5) or equation (6)respectively:

Z _(i)=[Z _(i)(1:Noc/2),zero(N−Noc),Z _(i)(N−Noc/2+1:N)]   equation (5)

when Noc is even.

Z _(i)=[Z _(i)(1:(Noc+1)/2),zero(N−Noc),Z_(i)(N−(Noc−1)/2+1:N)]  equation (6)

when Noc is odd.

In some embodiments, the transmitter 118 is not DC coupled and the DCFFT bin of a sequence Z_(i) is removed. The DC removal unit 204-iremoves the DC FFT bin of a sequence Z_(i) by causing the FFT bin at theindex 0 of the sequence Z_(i) to be null, i.e. Z_(i)(0)=0.

In some embodiments, as discussed above when the sequences are generatedin the frequency domain they are transformed by the IFFT units 205-1 to205-P into sequences z_(i) in the time domain. This can be performedfollowing the processing of the sequences Z_(i) in the power boost units202-i, the bandwidth limiters 203-i, and the DC removal units 204-i andprior to their processing in the cyclic prefix adders 206-i.

Following the conversion of the sequences Z_(i) into sequences z_(i)expressed in the time domain, a cyclic prefix (CP) is added to thesequences by the cyclic prefix adder 206-i. The cyclic prefix is a copyof last cyclic prefix samples in the time domain signal added to thebeginning of the signal (i.e., the beginning of the sequence z_(i)):

z _(i)=[z _(i)(N−CP+1:N),z _(i)(1:N)]  equation (7)

In some embodiments, the amplitude of sequences z_(i) is scaled to adefined power level.

Once the first set of sequences has been generated and processed, thefirst set of z_(i) sequences for i=1 . . . P are obtained and expressedin the time domain. A second set of sequences D_(i) (for i=1 . . . P) isgenerated at the OTSG 250 to be used in combination with the sequencesz_(i) to determine the P orthogonal training signals. In a similarmanner to the first set of first sequences z_(i), the second set ofsecond sequences D_(i) is generated based on a reference Zadoff-Chusequence D₁ and circular shift applied to this second reference sequenceto obtain the subsequent (P−1) second sequences.

The OSTC 250 includes a second reference Zadoff-Chu (ZC) sequencegeneration unit 209-1. The first reference ZC generation unit 201-1 isoperative to generate a second reference ZC sequence D₁ of lengthN_(eq), with parameters u₂ and q₂. In some embodiments, D1 is generatedaccording to the following equation (8):

$\begin{matrix}{D_{1} = e^{({{- j}\frac{\pi({{u_{2}{\lbrack{0:{N_{eq} - 1}}\rbrack}}{({{\lbrack{0:{N_{eq} - 1}}\rbrack} + 1 + {2q_{2}}})}}}{Neq}})}} & {{equation}\mspace{14mu} (8)}\end{matrix}$

where N_(eq) indicates a number of FFT bins selected for the sequenceD₁. In some embodiments, the number N_(eq) is a prime number that isgreater or equal to the number P of transmit paths in the transmitter118. The sequence D₁ is a signal generated at the second reference ZCgeneration unit 209-1 according to equation (8).

The sequence D₁ is used as a reference signal to generate additional(P−1) ZC sequences by cyclically shifting D₁ with a shift step size:Step_size=└N_(eq)/P┘, where └N_(eq)/P┘ is the floor function applied toN_(eq)/P, N_(eq) is a prime number that is greater or equal to thenumber P of transmit paths of the transmitter 118. The cyclic shift isapplied by each one of the Shift units 209-2 to 209-P to generate the(P−1) ZC subsequence sequences D_(i) based on the second reference D₁sequence. The cyclic shift between two consecutive sequences from thesecond set of sequences (where the second set of sequences includes thesecond reference sequence D₁ and the additional ZC sequences D_(i), fori=2 . . . P) is an integer that is smaller than or equal to the numberN_(eq) of FFT bins divided by the number P of transmit paths.

Thus, for each i an integer between 2 and P, the sequence D_(i) isgenerated based on the following equation (9):

D _(i)=circular_shift(D _(i) ,i*└N _(eq) /P┘).  equation (9)

Where └N_(eq)/P┘ is the floor function as applied to N_(eq)/P and theresult of the floor function is an integer that is smaller than or equalto the number N_(eq) of FFT bins divided by the number P of transmitpaths.

Once the second set of second sequences D_(i) has been generated it isused in combination with the first set of sequences z_(i) to determinethe training signals C(i) for i=1 . . . P.

The determination of the C(i) training signals is performed byperforming the following operations:

1) Replicate each one of the z_(i) sequences, for k=1 . . . P, N_(eq)times to obtain the concatenated signals: z₁z₁ . . . z₁, z_(P)z_(P) . .. z_(P), where each one of the z_(i) z_(i) z_(i) . . . z_(i) is formedby concatenating N_(eq) times the same sequence z_(i).

2) Multiply each concatenated sequence z_(i) . . . z_(i) by D_(i)(n),where n=1 . . . Neq. The multiplication of a concatenated sequence witha D_(i) sequence is performed by multiplying the entire frame of eachsequence z_(i) with a respective sample at index n of the respective Disequence. Thus, the entire FFT frame of each first ZC sequence z_(i) ismultiplied by one symbol D_(i)(n) of the second sequence D_(i) thusconstituting N_(eq) FFT frames of length N each. Every sequence z_(i)(i.e., the entire FFT frame forming the sequence z_(i)) from theconcatenated sequence z_(i) is multiplied by a different common phaseD_(i)(n). For example, the result of the multiplication for i=1 is:

z ₁(1:N)*D ₁(1)z ₁(1:N)*D ₁(2)z ₁(1:N)z ₁(1:N)*D ₁(N _(eq))  equation(10).

Where z₁(1:N) represents the entire FFT frame of the sequence z₁, andD₁(n) for n=1 . . . N_(eq), is the sample at index n for the sequenceD₁.

This operation is performed for each one of the first sequences z_(i)for i=1 . . . N resulting in P orthogonalal training signals asillustrated in FIG. 4. In some embodiments, where Z_(i) are generated inthe frequency domain, the multiplication z_(i)×D_(i)(n) can be done inthe frequency domain, i.e. before an IFFT is applied to the sequences.In these embodiments, the IFFT is applied to the set of P trainingsignals that result from the multiplication operation.

The P training signals obtained are orthogonal to one another in thetime domain as well as in the frequency domain. The P training signalscan be used to perform calibration and/or branch supervision of theantenna array. Orthogonality of the training signals in frequencydomain, indicates that the same FFT bins of different signalstransmitted across sub-arrays of the antenna array are orthogonal to oneanother. Orthogonality of the training signals in the time domain,indicates that the same FFT frame of different signals transmittedacross sub-arrays of the antenna array are orthogonal to one another.The generation of training signals as described with respect to FIG. 2and resulting in the P training signals shown in FIG. 3 allows fororthogonality of the signals with one another in the time domain as wellas in the frequency domain. To achieve the orthogonality in both domain,N and N_(eq) that are selected as prime numbers that are greater than orequal to the number P of transmit paths.

The operations in the flow diagram of FIGS. 4A-B will be described withreference to the exemplary embodiments of FIGS. 1, 2, and 3. However, itshould be understood that the operations of the flow diagrams can beperformed by embodiments of the invention other than those discussedwith reference to FIGS. 1, 2, and 3 and the embodiments of the inventiondiscussed with reference to these other figures can perform operationsdifferent than those discussed with reference to the flow diagrams ofFIGS. 4A-B.

FIG. 4A illustrates a flow diagram of exemplary operations performed forgenerating orthogonal training signal in accordance with a firstembodiment. The operations of the flow diagrams of FIG. 4A and FIG. 4Bare performed by the OTSG 250 that is an exemplary implementation of anOTSG 150 of the transmitter 118. In some embodiments, the OTSG 250 maybe part of a radio unit of the transmitter 118 or alternatively of thebaseband unit of the transmitter 118 without departing from the scope ofthe present invention. At operation 402, a set of P training signals aregenerated. The operation 402 includes the operations 404 to 412. Atoperation 404, the OTSG 250 generates a first reference Zadoff-Chusequence (e.g., the first ZC sequence can be generated in the frequencydomain Z₁ or in the time domain z₁). The first reference Zadoff-Chusequence is of length N, where N indicates a number of Fast FourierTransform (FFT) bins. N is a prime number that is greater than or equalto the number P of transmit paths. In some embodiments, the firstreference sequence is generated based on equation (1).

The flow of operations moves to operation 406, at which the OTSG 250generates (P−1) subsequent first Zadoff-Chu sequences (e.g., Z_(i)).Each one of the first subsequent Zadoff-Chu sequences is a cyclic shiftof the first reference Zadoff-Chu sequence, and a cyclic shift betweentwo consecutive sequences from the first set (where the first set ofsequences includes the first reference sequence and the first subsequentsequences) is an integer that is smaller than or equal to the number Nof FFT bins divided by the number P of transmit paths.

At operation 408, the OTSG 250 generates a second reference Zadoff-Chusequence D₁ of length N_(eq). N_(eq) indicates a number of FFT framesselected for the sequence D₁ and N_(eq) is a prime number that isgreater than or equal to the number P of transmit paths.

At operation 410, the OTSG 250 generates (P−1) second subsequentZadoff-Chu sequences (D_(i)). Each one of the second subsequentZadoff-Chu sequences is a cyclic shift of the second referenceZadoff-Chu sequence. The cyclic shift between two consecutive sequencesfrom the second set is an integer that is smaller than or equal to thenumber of FFT frames N_(eq) divided by the number P of sub-arrays. Thesecond reference Zadoff-Chu sequence and the (P−1) second subsequentZadoff-Chu sequences form a second set of P second sequences.

The flow of operations moves to operation 412, at which the OTSG 250determines the set of P training signals based on the first sequencesand the second sequences. In some embodiments, the determination of theP training signals is performed according to operations of FIG. 4B.

Once the P training signals are generated, each one of the P trainingsignals is transmitted, operation 414, through a transmit path from theplurality of transmit paths of the base station towards a wirelessnetwork. In some embodiments, in addition to being transmitted throughthe transmit paths, the P training signals are transmitted to animpairment estimator (e.g., impairment estimator 770 of FIG. 7) forenabling calibration of the antenna array. In other embodiments, the Ptraining signals may further be transmitted to a branch supervision unitthat is operative to determine the status of operation of the antennaarray. The orthogonality of the P training signals obtained in thefrequency domain and in the time domain enable a more accurate and moreefficient calibration and branch supervision processes.

FIG. 4B illustrates a flow diagram of exemplary operations that areperformed for determining the P training signals based on the first ZCsequences and the second ZC sequences in accordance with someembodiments.

At operation 434, the OTSG 250 replicates, for each sequence from thefirst set of sequences, the sequence N_(eq) times and concatenates thereplicated sequences to obtain a concatenated sequence. At operation436, the OTSG 250 multiplies each one of the concatenated sequences withN_(eq) samples of a respective second sequence. FIG. 3 illustratesexemplary P training signals obtained as a result of the replicationoperation 434 and the multiplication operation 436.

FIG. 4C is a flow diagram of exemplary operations that can be performedduring generation of the P training signals in accordance with someembodiments. In some embodiments, prior to determining the P trainingsignals based on the first set of sequences {z_(i), i=1 . . . P} and asecond set of sequences {D_(i), i=1 . . . P} several operations can beperformed on each one of the first sequences z_(i). For example, theseadditional operations are performed for each sequence z_(i) by arespective one of the power boost units 202-i, the bandwidth limiters203-i, the DC removal units 204-i, the cyclic prefix adder 206-i, andthe power normalizer 207-i. In some embodiments, none of theseoperations is performed. In other embodiments, a subset or all theoperations are performed without departing from the scope of the presentinvention.

In the following description, the operations will be described withrespect to a single sequence Zi, however one of ordinary skill in theart would understand that the same operations are performed for each oneof the first sequences Zi, for i=1 . . . P. At operation 422, the powerlevel of at least one of the FFT bins of the sequence Zi is boosted.Boosting the power level of improves the signal to noise ratio of thosebins. For example, in some embodiments, the FFT bins of the sequencethat are close to the edges of the bandwidth occupied (Noc) are boosted.

At operation 424, the frequency band of the sequence is limited based onthe Noc. As mentioned above, Noc indicates the number of FFT bins of thesequence that are within the bandwidth of a digital transmit path. Thenumber Noc is usually narrower than the Nyquist bandwidth. In a firstembodiment, when the sequence z_(i) is generated in the time domain, thefrequency band can be limited by applying a digital filter with adesired bandwidth in the time domain. Alternatively, when the sequenceZ_(i) is generated in the frequency domain, the frequency band can belimited by applying a digital filter with a desired bandwidth in digitalfrequency domain. For example, this can be performed by nullifying FFTbins of the sequence Zi that are outside of the occupied bandwidth andkeeping the remaining of the FFT bins unchanged.

In some embodiments, the bandwidth limited 203-i limits the frequencyband of the sequence Z_(i) for i=1 . . . P to contain Noc occupied FFTbins according to the following equations:

The Noc is determined based on the following equation (4)

Noc=┌Carrier_Bandwith*s/Nyquist Sampling Rate*N)┐  equation (4),

where the carrier_bandwitdh is the bandwidth of the carrier wave of thetransmit path, and s is a parameter that can have a value of 0.9. Othervalues of s can be contemplated.

Depending on whether Noc is even or odd, the sequence frequency band ofthe sequence Zi is limited according to equation (5) or equation (6)respectively:

Z _(i)=[Z _(i)(1:Noc/2),zero(N−Noc),Z _(i)(N−Noc/2+1:N)]  equation (5)

when Noc is even.

Z _(i)=[Z_i(1:(Noc+1)/2),zero(N−Noc),Z _(i)(N−(Noc−1)/2+1:N)]  equation(6)

when Noc is odd.

Flow then moves to operation 426, at which DC FFT bin of a sequenceZ_(i) is removed. The DC removal unit 204-i removes the DC FFT bin of asequence Z_(i) by causing the FFT bin at the index 0 of the sequenceZ_(i) to be null, i.e. Z_(i)(0)=0.

In some embodiments, as discussed above when the sequences are generatedin the frequency domain, the flow of operations moves to operation 428,at which each one of the first sequences is transformed into sequencesz_(i) in the time domain.

At operation 430, a cyclic prefix (CP) is added to the sequences. Thecyclic prefix is a copy of last cyclic prefix samples in the time domainsignal added to the beginning of the signal (i.e., the beginning of thesequence z_(i)). At operation 432, the power of the sequences z_(i) isnormalized to a nominal amplitude value (e.g. +/−1), i.e., the amplitudeof sequences z_(i) is scaled to a defined power level.

Orthogonality of the Training Signals in Frequency Domain for AntennaArray Calibration:

In some embodiments, the P training signals can be generated such thatorthogonality in a single domain is achieved. FIG. 5 illustrates a blockdiagram of an exemplary OTSG 560 that is operative to generate Ptraining signals that are orthogonal to one another in the frequencydomain. In this embodiments, the first reference ZC sequence generationunit 501 is used in addition to a set of second sequences to generatethe P training signals. The set of second sequences is generated basedon a second reference ZC sequence, generated at the generation unit509-1 and subsequent ZC sequences that are generated based on a cyclicshift of the second reference sequence. In some embodiments, theoperation performed for generating the first reference sequence at thefirst reference ZC sequence generation unit 501 are similar to operationperformed with reference to the first reference ZC generation unit 201-1of FIG. 2. The operation performed for generating the second referencesequence at the second reference ZC sequence generation unit 509-1 aresimilar to operations performed with reference to the second referenceZC generation unit 209-1 of FIG. 2. The generation of the subsequentsecond sequences is performed based on similar operations as discussedwith reference to FIG. 2 and the generation of the cyclic shifts of thesecond reference signal.

In these embodiments, the orthogonality in the frequency domain of the Ptraining signals (i.e. orthogonality of the signals between the same FFTbins of these sequences across antenna array) is obtained at least byselecting an N_(eq) used for generation of the second ZC sequences thatis a prime number greater than or equal to the number of transmit pathin the base station. In addition, to selecting the N_(eq), the parameteru₂ of the second ZC sequence is selected to be an integer between 0 andN_(eq) (i.e., 0<u₂<N_(eq)). In some embodiments, the orthogonality ofthe P training signals in the frequency domain allows a more efficientcalibration of the antenna array when using the P training signals.

Orthogonality of the Training Signals in Time Domain for Antenna ArrayCalibration:

In some embodiments, the P training signals can be generated such thatorthogonality in a single domain is achieved. FIG. 6 illustrates a blockdiagram of an exemplary OTSG 660 that is operative to generate Ptraining signals that are orthogonal to one another in the time domain.In this embodiments, the first set of ZC sequences is generated based ona first reference ZC sequence (generated by the generation unit 601-1)and subsequent ZC sequences that are generated based on a cyclic shiftof the first reference sequence. In this embodiment, the operationperformed for generating the first reference sequence at the firstreference ZC sequence generation unit 601-1 are similar to operationperformed with reference to the first reference ZC generation unit 201-1of FIG. 2. The operation performed for generating the subsequence ZCsequences at the shift units 601-2 to 601-P are similar to operationsperformed with reference to the first reference ZC generation unit 209-1of FIG. 2.

In these embodiments, the orthogonality in the time domain of the Ptraining signals (i.e. orthogonality of the signals between the same FFTframe of these sequences across antenna array) is obtained at least byselecting an N used for generation of the first ZC sequences that is aprime number greater than or equal to the number of transmit path in thebase station. In addition, to selecting the N, the parameter u₁ of thefirst ZC sequence is selected to be an integer between 0 and N (i.e.,0<u₁<N). In some embodiments, the orthogonality of the P trainingsignals in the frequency domain allows a more efficient branchsupervision of the transmit paths coupled with sub-arrays of the antennaarray when using the P training signals.

Using the Orthogonal Training Signals for Antenna Array Calibrationand/or Branch Supervision:

In some embodiments, the orthogonal training signals are generated andtransmitted through the transmit paths of a base station towardssub-array of an antenna array to allow for branch supervision (i.e.,fault detection) and/or calibration of the transmit paths. Thecalibration and/or branch supervision can be performed at regularintervals during the operation of the antenna array. In someembodiments, calibration of the antenna array enables at regularinterval allows the system to account for changes in the environmentthat may affect the system (e.g., weather conditions, amount of datatraffic transmitted through the antenna array, etc.).

In some embodiments, the calibration can be performed entirely withinthe radio unit of a base station using a feedback signal from anantenna, without involving other parts of the base station and thenetwork. Confining the calibration within the radio unit can simplifythe hardware and software design and lower the cost of the system.Moreover, the radio unit in a base station is typically multi-standard,which also means that the radio unit is agnostic to the specific radiostandard (i.e., 5th generation wireless systems (5G), 4th generationwireless systems (4G), Long Term Evolution (LTE), Global System forMobile Communications (GSM), Code Division Multiple Access (CDMA),Wideband CDMA (WCDMA), etc.) being implemented by the base station.Therefore, performing the calibration in the radio unit maintains themulti-standard characteristic of the base station. It is to beappreciated that embodiments of the invention are not so limited; insome embodiments, the calibration can be performed by the radio unit andother parts of the base station, e.g., the baseband unit. For example,the generation of the signals can be performed in the radio unit oralternatively in the baseband unit.

FIG. 7 illustrates an exemplary block diagram of a portion of thetransmitter 114 in the base station 110 that performs antenna arraycalibration and/or branch supervision according to one embodiment. Inthis embodiment, the transmitter 114 includes a baseband unit 712 and aradio unit 710 coupled to an antenna array 118 that has multiplesub-arrays 721(1-P). The radio unit 710 is responsible for convertingbaseband signals into radio frequency (RF) signals for transmission. Thesub-arrays 721(1-P) are operative to carry outbound signals that havebeen phase-controlled for transmission. The outbound signals includetraffic signals. Each of the traffic signals is a “normal trafficsignal” as the signal carries data or other communication informationfor transmission to another network node or user equipment. In someembodiments, the outbound signals may include traffic signals andtraining signals that were injected into the base station 110 forperforming antenna array calibration. The sub-arrays 721(1-P) arecoupled to the radio unit 710 of the base station 110 via respectiveantenna ports and corresponding radio transmit ports at the radio unit710 (not illustrated).

Between the antenna ports and the radio transmit ports are multiplefeeders (not illustrated), one for each transmit path. The term“transmit path” as used herein refers to the path traversed by a signalafter the signal enters one of the transmit (Tx) chains 730(1-P), forexample Tx Chain 730-1, and before the signal enters one of thesub-arrays 721(1-P). An example of a transmit path is shown in FIG. 7 bythe dotted box labeled as a transmit path 729-1. The transmit path 729-1includes a transmit (Tx) chain 730-1 and all of the interconnectincluding a feeder (not illustrated) up to a coupler 723-1 inside theantenna array 118. In practice the transmit path 729-1 may also includeduplexers, amplifiers (e.g., tower mounted amplifiers (TMAs), combiners,diplexers, etc., such as would be appreciated by one skilled in the art.There is a one-to-one correspondence between a transmit chain and atransmit path. The transmit chains 730(1-P) are the boundary betweendigital processing and analog processing in the base station 110, aseach one of the transmit chains 730(1-P) converts a signal from digitalto analog. Each one of the transmit chain 730(1-P) includes a number ofanalog components, such as one or more digital-to-analog converters,mixers, filters, power amplifiers, etc.

The analog components in the transmit chains 730(1-P), together with thefeeders and other components along the analog portion of the transmitpaths up to the antenna ports, generally cause linear phase and/orlinear amplitude impairment to the signals that traverse these paths.Significant non-linearities in the transmit path (such as the poweramplifier) are typically taken care of by non-linear pre-distortiontechniques.

The base station includes an orthogonal training signal generator 750that is operative to generate P orthogonal training signals. The Ptraining signals can be generated as described above with respect to themultiple embodiments of FIGS. 2-6. For example, the P training signalscan be generated in the frequency domain as signals C(1,m, k), . . . ,C(P,m, k), where (FFT frame) index is m={1, 2, . . . , N_(eq)}, sampleindex is n and k is frequency index. The P training signals can also begenerated in the time domain as signals c(1, m, k) . . . c(P, m, k). Inaddition, the P training signals can be generated such orthogonality inat least one of the time domain and the frequency is achieved. In someembodiments, the orthogonality is achieved in a single one of the timeand frequency domain. In other embodiments, orthogonality is achieved inboth domains.

The P training signals are input into the radio unit 710 to betransmitted through the respective transmit paths 729-1 to 729-P. Eachof the input signals c(i, m, n), where n is the time index, is a signalto be transmitted to a wireless network via one of the sub-arrays721(1-P) by traversing a respective transmit path from the transmitpaths 729(1-P). In the description herein, the lower-case lettersindicate time-domain signals or values, and the upper-case lettersindicate frequency-domain signals or values.

To determine the linear impairment of phase and/or amplitude affectingsignals in the transmit paths, input signals C(1,m, k), . . . , C(P,m,k)) are transmitted to the impairment estimator 770 when these signalsare transmitted to the radio unit 710. The P training signals aresampled in m blocks of N samples per block. The signals, C(1,m, k), . .. , C(P,m, k) when entering the impairment estimator 770 have not beenimpaired by the components in the transmit paths, and, therefore, aresuitable for determining reference signals for performing thecalibration and/or branch supervision.

The training signals traverse the different components of thetransmitter (e.g., the conditioning units, the equalizers, the Txchains, the feeders) to be output as outbound signals at the subarrays721(1-P) for transmission towards a wireless network. The outboundsignals are coupled by respective couplers 723(1-P) and combined (i.e.,summed up) by a combiner 722 in the antenna array 118 to produce asingle feedback signal s_(dt)(n). This feedback signal is routed to afeedback receiver 740 through an antenna calibration port (notillustrated) and a corresponding radio calibration port (notillustrated) at the radio unit 710.

The feedback signal s_(dt)(n) is formed after each input signal has gonethrough the various components of the transmitter, in particular theanalog part of the transmit path. Thus, the feedback signal, s_(dt)(n),is a sum of the impaired signals. The calibration technique describedherein uses the unimpaired reference signals C(1,m, k), . . . , C(P,m,k) and the impaired sum of these signals to estimate the impairment inthe transmit path and to thereby remove the impairment from the outboundsignals that is output at the antenna subarrays 721 (1-P) during acalibration process or to detect a fault in the transmit paths during abranch supervision process. In a calibration process, the impairment isremoved after the equalizers 790(1-P) are programmed with equalizer tapscalculated based on the estimated impairments at the equalizer synthesisunit 780. In some cases, only the differences in the impairments of thetransmit paths need to be compensated for to obtain good systemperformance.

The feedback signal s_(dt)(n) from the combiner 722 is sent to thefeedback receiver 740, which removes the cyclic prefix from the feedbacksignal to obtain the new feedback signal s_(F) (m, n). An FFT of thesignal s_(F)(m, n) is taken to obtain the signal s_(F)(m, k). Theimpairment estimator 770 is operative to determine, based on thereference signals and the feedback signal, the impairment affecting theinput signals in the transmit path.

The impairment estimator 770 aligns the feedback signal with thereference signals in time, and performs a de-convolution of thereference signals jointly with the feedback signal. The result of thede-convolution is an estimated impairment for each transmit path. As theeffect of impairment is equivalent to convolving the reference signalswith the impairment, the impairment may be calculated by de-convolvingthe reference signals with the impaired feedback signal.

In the scenario of a calibration process, based on the estimatedimpairment determined at the impairment estimator 770, an equalizersynthesis unit 780 computes an approximate inverse to the impairment inthe frequency range occupied by the outbound traffic signals. Theequalizer synthesis unit 780 produces a set of equalizer tapsrepresentative of the approximate inverse to the impairment. Theequalizer synthesis unit 780 determines and sets the tap values of thecorresponding equalizers 790(1-P) according to the equalizer taps. Inone embodiment, each one of the equalizers 790(1-P) is a complex finiteimpulse response (FIR) filter with one or more taps (i.e., equalizertaps). The finite impulse response is an approximate inverse to theimpairment that occurs in the corresponding transmit path from atransmit chain 730 to the antenna port that couples the radio unit 710to the antenna array 118. As such, each outbound traffic signalprocessed by one of the equalizer 790(1-P) is pre-distorted such thatthe pre-distortion cancels out the impairment in the transmit path.

Although FIG. 7 shows that the calibration functions are performedentirely in the radio unit 710, some or all of the calibration functionscan be performed in the baseband unit 712 of the base station 110. Insome embodiments, the calibration functions may be performed in theradio unit 710, the baseband unit 712, and/or other portions of the basestation 110.

In the scenario of branch supervision, based on the estimated impairmentdetermined at the impairment estimator 770, a transmit path faultmanager 772 estimates performance metrics related to each one of thetransmit path and may determine whether or not a fault has occurred inthe transmit path. In some embodiments, the transmit fault manager 772is operative to transmit results of the analysis to a higher levelentity such as an administrator of the system (e.g., through a backendnetwork and a graphical user interface) to inform of the fault thatoccurred in the transmit path(s).

In one embodiment, the impairment estimation and equalization areperformed in a continuous loop, where the feedback signal and trainingsignals are continuously captured over time and are continuously used torefine the equalizer taps and perform the branch supervision. Thecomputation of impairment estimation and equalization can be performedoffline or in real-time. For example training signals transmitted over aperiod of time may be used in offline processing in order to obtain anaccurate impairment estimation and equalization. Alternatively,real-time processing may be more responsive to changes in operatingconditions. In some embodiments, the base station 110 may dynamicallyswitch between offline and real-time processing based on the currentoperating conditions. In some embodiments, the P training signals can beinjected into the transmit branches at regular intervals (e.g., every 10sec to perform branch supervision and/or branch calibration). In someembodiments, the branch supervision can be performed on differentintervals than the calibration. For example, calibration can beperformed more often than branch supervision and vice-versa.

FIG. 8 illustrates a flow diagram of exemplary operations for using thetraining signals to perform calibration and/or branch supervision of thetransmit paths of a base station in accordance with some embodiments.The operations in the flow diagram can be performed by embodimentsdescribed with respect to FIGS. 1-7. However, it should be understoodthat the operations of the flow diagram can be performed by embodimentsof the invention other than those discussed with reference to FIGS. 1-7and the embodiments of the invention discussed with reference to theseother figures can perform operations different than those discussed withreference to the flow diagram of FIG. 8.

At operation 802, each of one of P training signals is transmitted overa respective one of multiple transmit paths of a base station. Thetraining signals are orthogonal to one another in at least one of thefrequency and the time domains. In some embodiments, the trainingsignals are orthogonal to one another in both the frequency and the timedomain. In some embodiments, the training signals result, operation 804,from a multiplication a first set of ZC sequences of length N and asecond set of ZC sequences of length N_(eq), where each of N and N_(eq)is a prime number that is greater than or equal to the number P oftransmit paths. In some embodiments, the training signals are generatedwithin the base stations (within the radio unit or alternatively withinthe baseband unit), for example with an orthogonal training signalgenerator (e.g., OTSG 250, 550, or 650). In other embodiments, thetraining signals can be generated outside the base station and stored atthe base station to be transmitted for performing calibration and/orbranch supervision. In these examples, the OTSG can be located on anetwork device coupled with the base station and that is operative totransmit the generated training signals to be stored at the basestation. While the embodiments, described above with reference to FIG. 1illustrates an OTSG 150 that is located within the base station 110,this should not be regarded as a limitation of the present invention. Insome embodiments, the oTSG may be outside the base station and coupledto the base station through a network. In these embodiments, the basestation is operative to store the generated training signals and usethem for performing any of a calibration process and/or branchsupervision process of the antenna array.

At operation 806, a feedback signal is received. For example, thefeedback signal is received at the impairment estimator 770. Thefeedback signal is a combination of the P training signals as capturedafter having traversed the plurality of transmit paths and prior tobeing transmitted at the plurality of sub-arrays of an antenna array. Atoperation 808, for each transmit path of the base station a respectiveimpairment function is determined based on the feedback signal and the Ptraining signals. The orthogonality of the training signals in at leastone of the time domain and the frequency domain enable to perform anefficient calibration and/or branch supervision.

The impairment function can be used to enable calibration and/or branchsupervision of the transmit paths coupled with the sub-arrays of theantenna array. In some embodiments, the flow of operations moves tooperation 810, at which the impairment function determined for each oneof the plurality of transmit paths is used for calibration of thesepaths. In other embodiments, the flow of operations moves to operation812, at which the impairment function is used to supervise the antennaarray and detect faulty transmit paths from the plurality of transmitpaths. In some embodiments, the branch supervision can be performed byassessing, at operation 814, one or more performance metrics, such aspower, noise floor, Error Vector Magnitude (EVM), Signal to Noise Ratio(SNR), etc. The branch supervision further includes detecting, atoperation 816, based on at least one of the performance metrics a faultat a transmit path.

Impairment function based on the feedback signal the and P orthogonaltraining signals:

The operations below will be described with reference to FIG. 7 with theassumption that the number of transmit paths P=4. Upon receipt of thefeedback signal S_(F)(m,n) and the training signals C(1,m,k) . . .C(4,m.k), the impairment estimator 770 determines linear equations foreach bin k, FFT frame index m.

The linear equation for each frequency bin k, FFT frame index m and P=4can be written as follows:

C(1,m,k)H _(d)(1,k)+C(2,m,k)H _(d)(2,k)+C(2,m,k)H _(d)(2,k)+C(3,m,k)H_(d)(3,k)+C(4,m,k)H _(d)(4,k)=S _(F) k)

where H_(d) (i, k) is the transfer function in the frequency domainbetween the training signals and the feedback signal. The number of FFTframes N_(eq) determines the number of linear equations.

This linear equation has four unknown coefficients H_(d) (i, k) for theP transmit paths (that is, one impairment value for each transmit path).In order to solve for H_(d)(i, k), four or more independent equationsare needed, when P=4. This can be achieved by using different FFT frameswith index m={1, 2, . . . , 4}. The equations in matrix form for eachfrequency bin k is as follows:

C(k)H_(d)(k) = S_(F)(k)  where ${C(k)} = \begin{bmatrix}{C\left( {1,1,k} \right)} & \ldots & {C\left( {4,1,k} \right)} \\{C\left( {1,2,k} \right)} & \ddots & {C\left( {4,2,k} \right)} \\\vdots & \; & \vdots \\{C\left( {1,M,k} \right)} & \ldots & {C\left( {4,M,k} \right)}\end{bmatrix}$ ${H_{d}(k)} = \begin{bmatrix}{H_{d}\left( {1,k} \right)} \\{H_{d}\left( {2,k} \right)} \\\vdots \\{H_{d}\left( {4_{,}k} \right)}\end{bmatrix}$ ${S_{F}(k)} = \begin{bmatrix}{S_{F}(1,k)} \\{S_{F}\left( {2,k} \right)} \\\vdots \\{S_{F}\left( {4,k} \right)}\end{bmatrix}$

The estimated impairment vector for each frequency bin k is then, H_(d)(k)=C⁺(k)S_(F) (k) where C⁺(k)=[C^(H)(k)C(k)]⁻¹C^(H)(k) is the pseudoinverse of C and H is the Hermitian (conjugate transpose). Theorthogonality of the training signals in the time domain and in thefrequency domain causes C^(H)(k)C(k)=I, where I is the identity matrix.Thus C⁺(k)=[C^(H)(k)C(k)]⁻¹C^(H)(k) simplify to C⁺(k)=C^(H)(k). Theimpairment estimation is then:

H _(d)(k)=C ^(H)(k)S _(F)(k).

Thus, as shown above the determination of the impairment function isgreatly simplified thus saving significant computational and storageresources at the impairment estimator which can enable a fastercalibration and branch supervision of the transmit paths of the basestation.

The embodiments of the present invention described herein enablecalibration techniques and branch supervision techniques that haveseveral advantages when compared with existing calibration techniques.The use of the orthogonal training signals the inversion of matrix orpseudo inversion of matrix that was previously required for the solutionof the transfer function is eliminated saving computation time andmemory. Further, only two sets of Zadoff-Chu sequences are used to solvethe transfer function of all the transmit paths which greatly simplifiesthe implementation of the impairment function estimation and minimizethe memory needed. The orthogonality of the training signals providesimproved measures of signal-to-interference-plus-noise ratio (SINR) forthe estimation of the transfer function. The set of training signals canbe used for both antenna calibration and branch supervision. Each of thetraining signals has a low peak to average power ratio (i.e. the signalsare well behaved time response for use in a transmit chains). In someembodiments, the training signals are generated in the frequency domain,which ensures a good signal level for a transfer function estimationacross frequency.

In one embodiment, the calibration and branch supervision can beperformed entirely within the radio unit of a base station using afeedback signal from an antenna and the orthogonal training signals,without involving other parts of the base station and the network.Confining the calibration and/or branch supervision within the radiounit can simplify the hardware and software design and lower the cost ofthe system. Moreover, the radio unit in a base station is typicallymulti-standard, which also means that the radio unit is agnostic to thespecific radio standard (i.e., 5th generation wireless systems (5G), 4thgeneration wireless systems (4G), Long Term Evolution (LTE), GlobalSystem for Mobile Communications (GSM), Code Division Multiple Access(CDMA), Wideband CDMA (WCDMA), etc.) being implemented by the basestation. Therefore, performing the calibration in the radio unitmaintains the multi-standard characteristic of the base station. It isto be appreciated that embodiments of the invention are not so limited;in some embodiments, the calibration can be performed by the radio unitand other parts of the base station, e.g., the baseband unit. However,the digital circuitry in the baseband unit is generally multi-standardin hardware, but typically uses unique software and configurationspecific to each radio standard. Moreover, performing the calibration inthe radio unit and the baseband unit may incur additional interconnectsand coordination between these two units. Therefore, performing thecalibration in the baseband unit or multiple units of the base stationmay be more costly than performing the calibration entirely in the radiounit. The calibration technique described herein may be implemented inboth time-division duplex (TDD) and frequency-division duplex (FDD)systems.

Architecture:

Different embodiments of the invention may be implemented usingdifferent combinations of software, firmware, and/or hardware. Thus, thetechniques shown in the figures can be implemented using code and datastored and executed on one or more electronic devices (e.g., an endstation, a network device). Such electronic devices store and transmit(internally and/or with other electronic devices over a network) code(composed of software instructions) and data using computer-readablemedia, such as non-transitory tangible computer-readable media (e.g.,computer-readable storage media such as magnetic disks; optical disks;read only memory; flash memory devices) and transitory computer-readabletransmission media (e.g., electrical, optical, acoustical or other formof propagated signals such as carrier waves, infrared signals). Inaddition, such electronic devices typically include a set of one or moreprocessors coupled to one or more other components, such as one or morenon-transitory machine-readable media (to store code and/or data), userinput/output devices (e.g., a keyboard, a touchscreen, and/or adisplay), and network connections (to transmit code and/or data usingpropagating signals). The coupling of the set of processors and othercomponents is typically through one or more busses and bridges (alsotermed as bus controllers). Thus, a non-transitory computer-readablemedium of a given electronic device typically stores instructions forexecution on one or more processors of that electronic device. One ormore parts of an embodiment of the invention may be implemented usingdifferent combinations of software, firmware, and/or hardware.

In some embodiments, the generation of the training signals can beperformed on a dedicated hardware component such as a Field-ProgrammableGate Array (FPGA) or an application-specific integrated circuit (ASIC),or a general purpose processor. The generation of the orthogonaltraining signals can be performed within or outside of a base station.

As used herein, a network device (e.g., a router, switch, bridge,controller, base station) is a piece of networking equipment, includinghardware and software that communicatively interconnects other equipmenton the network (e.g., other network nodes, user equipment, etc.). Somenetwork nodes are “multiple services network nodes” that provide supportfor multiple networking functions (e.g., routing, bridging, switching,Layer 2 aggregation, session border control, Quality of Service, and/orsubscriber management), and/or provide support for multiple applicationservices (e.g., data, voice, and video).

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described; it can be practiced withmodification and alteration within the scope of the appended claims. Thedescription is thus to be regarded as illustrative instead of limiting.

1. A method in a base station including a plurality of P transmit pathscoupled with a plurality of sub-arrays of an antenna array fortransmitting signals to a wireless network, the method comprising:generating a set of P training signals, wherein generating the set of Ptraining signals includes: generating a first reference Zadoff-Chusequence, wherein the first reference Zadoff-Chu sequence is of lengthN, wherein N indicates a number of Fast Fourier Transform (FFT) bins andN is a prime number that is greater than or equal to the number P oftransmit paths; generating (P−1) first subsequent Zadoff-Chu sequences,wherein each one of the first subsequent Zadoff-Chu sequences is acyclic shift of the first reference Zadoff-Chu sequence, and wherein acyclic shift between two consecutive sequences from the first set is aninteger that is smaller than or equal to the number N of FFT binsdivided by the number P of transmit paths, wherein the first referenceZadoff-Chu sequence and the (P−1) first subsequent Zadoff-Chu sequencesform a first set of P first sequences, generating a second referenceZadoff-Chu sequence of length Neq, wherein Neq indicates a number of FFTframes and Neq is a prime number that is greater than or equal to thenumber P of transmit paths, generating (P−1) second subsequentZadoff-Chu sequences, wherein each one of the second subsequentZadoff-Chu sequences is a cyclic shift of the second referenceZadoff-Chu sequence, and wherein a cyclic shift between two consecutivesequences from the second set is an integer that is smaller than orequal to the number of FFT frames Neq divided by the number P oftransmit paths, wherein the second reference Zadoff-Chu sequence and the(P−1) second subsequent Zadoff-Chu sequences form a second set of Psecond sequences, and determining the set of P training signals based onthe first set of first sequences and the second set of second sequences,wherein the P training signals are orthogonal to one another in the timeand the frequency domains; and transmitting each one of the P trainingsignals through a transmit path from the plurality of transmit paths ofthe base station towards a wireless network.
 2. The method of claim 1,wherein determining the set of P training signals includes: for eachfirst sequence from the first set, replicating the sequence Neq timesand concatenating the replicated sequences to obtain a concatenatedsequence; and multiplying each one of the concatenated sequences withNeq samples of a respective second sequence from the second set.
 3. Themethod of claim 1 further comprising: for each sequence from the firstset of sequences, boosting the power level of at least one of FFT binsof the sequence.
 4. The method of claim 1 further comprising: limiting afrequency band of each one of the first sequences to contain Nococcupied FFT bins, wherein the Noc occupied FFT bins indicates thenumber of FFT bins of the sequence that are within the bandwidth of atransmit path.
 5. The method of claim 1 further comprising: applying aninverse FFT (IFFT) onto each one of the first sequences over the N FFTbins to obtain a set of first sequences in a time domain.
 6. The methodof claim 1, further comprising: adding a cyclic prefix to each one ofthe first sequences.
 7. The method of claim 1, further comprising:normalizing the power of each of the first sequences to a nominalamplitude value.
 8. The method of claim 1, further comprising: receivinga feedback signal, wherein the feedback signal is a combination of the Ptraining signals as captured after having traversed the plurality oftransmit paths and prior to being transmitted at the plurality ofsub-arrays; and determining for each transmit path of the base station arespective impairment function based on the feedback signal and the Ptraining signals.
 9. The method of claim 8, further comprising using theimpairment function for each one of the plurality of transmit paths forcalibration of the antenna array.
 10. The method of claim 8, furthercomprising: using the impairment function to supervise the antenna arrayand detect faulty transmit paths from the plurality of transmit paths.11. A base station including a plurality of P transmit paths coupledwith a plurality of sub-arrays of an antenna array for transmittingsignals to a wireless network, the base station comprising: anorthogonal training signal generator operative to: generate a set of Ptraining signals, wherein generating the set of P training signalsincludes: generate first reference Zadoff-Chu sequence, wherein thefirst reference Zadoff-Chu sequence is of length N, wherein N indicatesa number of Fast Fourier Transform (FFT) bins and N is a prime numberthat is greater than or equal to the number P of transmit paths;generate (P−1) first subsequent Zadoff-Chu sequences, wherein each oneof the first subsequent Zadoff-Chu sequences is a cyclic shift of thefirst reference Zadoff-Chu sequence, and wherein a cyclic shift betweentwo consecutive sequences from the first set is an integer that issmaller than or equal to the number N of FFT bins divided by the numberP of transmit paths, wherein the first reference Zadoff-Chu sequence andthe (P−1) first subsequent Zadoff-Chu sequences form a first set of Pfirst sequences, generate a second reference Zadoff-Chu sequence oflength Neq, wherein Neq indicates a number of FFT bins frames and Neq isa prime number that is greater than or equal to the number P of transmitpaths, generate (P−1) second subsequent Zadoff-Chu sequences, whereineach one of the second subsequent Zadoff-Chu sequences is a cyclic shiftof the second reference Zadoff-Chu sequence, and wherein a cyclic shiftbetween two consecutive sequences from the second set is an integer thatis smaller than or equal to the number of FFT frames Neq divided by thenumber P of transmit paths, wherein the second reference Zadoff-Chusequence and the (P−1) second subsequent Zadoff-Chu sequences form asecond set of P second sequences, and determine the set of P trainingsignals based on the first set of first sequences and the second set ofsecond sequences, wherein the P training signals are orthogonal to oneanother in the time and the frequency domains; and transmit each one ofthe P training signals through a transmit path from the plurality oftransmit paths of the base station towards a wireless network.
 12. Thebase station of claim 11, where to determine the set of P trainingsignals includes: for each first sequence from the first set, replicatethe sequence Neq times and concatenate the replicated sequences toobtain a concatenated sequence; and multiply each one of theconcatenated sequences with Neq samples of a respective second sequencefrom the second set.
 13. The base station of claim 11, wherein theorthogonal training signal generator is further operative to: for eachsequence from the first set of sequences, boost the power level of atleast one of FFT bins of the sequence.
 14. The base station of claim 11,wherein the orthogonal training signal generator is further operativeto: limit a frequency band of each one of the first sequences to containNoc occupied FFT bins, wherein the Noc occupied FFT bins indicates thenumber of FFT bins of the sequence that are within the bandwidth of atransmit path.
 15. The base station of claim 11, wherein the orthogonaltraining signal generator is further operative to: apply an inverse FFT(IFFT) onto each one of the first sequences over the N FFT bins toobtain a set of first sequences in a time domain.
 16. The base stationof claim 11, wherein the orthogonal training signal generator is furtheroperative to: add a cyclic prefix to each one of the first sequences.17. The base station of claim 11, wherein the orthogonal training signalgenerator is further operative to: normalize the power of each of thefirst sequences to a nominal amplitude value.
 18. The base station ofclaim 11, wherein the orthogonal training signal generator is furtheroperative to: receive a feedback signal, wherein the feedback signal isa combination of the P training signals as captured after havingtraversed the plurality of transmit paths and prior to being transmittedat the plurality of sub-arrays; and determine for each transmit path ofthe base station a respective impairment function based on the feedbacksignal and the P training signals.
 19. The base station of claim 18,wherein the orthogonal training signal generator is further operative touse the impairment function for each one of the plurality of transmitpaths for calibration of the antenna array.
 20. The base station ofclaim 18, wherein the orthogonal training signal generator is furtheroperative to use the impairment function to supervise the antenna arrayand detect faulty transmit paths from the plurality of transmit paths.